FIG. 1 illustrates schematically, in cross-section, a mobile lens unit 100 comprising a cylindrical lens barrel 102 with a number of fixed lenses 104, 106 and 108 mounted therein, the lens barrel 102 having a conducting coil 110 mounted within its walls. The coil 110 is formed of a wire represented in cross-section by small circles with dots to represent a current flow one way in the wire, and crosses to represent a current flow in the opposite direct. Mobile lens unit 100 also comprises a housing 112, which is, for example, formed of plastic, and has rims 113, 114 at the top and bottom respectively, which extend some way towards the center of the housing, and act as stoppers for the lens barrel 102. A permanent magnet 115, which is for example also cylindrical, is mounted against the inner surface of the housing 112 and lies adjacent to the lens barrel 102. The coil 110, and the permanent magnet 115 together form a voice coil motor (VCM). Springs 116 and 118 are connected between the housing 112 and the lens barrel 102.
In operation, the mobile lens unit 100 is mounted over an image sensor (not shown), and the positioning of the lenses with respect to the image sensor can be adjusted by moving the lens barrel 102 up and down within the housing, which can be achieved by passing a current through the coil 110. Springs 116, 118 hold the lens barrel 102 at an initial position at the bottom end of the housing resting against rim 113 when no current is applied to the coil. When current is applied to the coil, due to electromagnetic force generated by the current flow in a magnetic field, the lens barrel moves towards the top of the housing, counteracting the force of the springs 116, 118. Springs 116, 118 have an increasing restoring force the further the lens barrel is from its initial start position, meaning that the lens barrel rests at a particular position depending on the current level applied to the coil.
A problem with the mobile lens unit 100 of FIG. 1 is that it can be very difficult to estimate the required current level to move the lens barrel to a determined lens position. This is because the current level for a given lens position depends on various factors, such as the orientation of the mobile lens unit with respect to gravity, the strength of the springs holding the lens barrel, the force of the springs at the initial position of the lens barrel and any friction in the system. Thus the same current may result in different lens positions on different occasions.
A proposed solution to this problem is to use a feedback loop attached to a position sensor, for example, formed of a laser or LED and a light detector fitted within the mobile lens unit, and designed to detect the position of the lens barrel within the housing. However, such systems are disadvantageous as they require relatively bulky components and additional circuitry, which is not compatible with most lens unit designs in which space is very limited, and the components are relatively high cost.